Lung cancer is the leading cause of cancer death and is estimated to claim over 160,000 lives in the United States in 2008; thus, the study of lung tumor cell biology is critical for discovering novel targets to treat the disease. Recent evidence suggests that solid tumors comprise a small fraction of lung cancer stem cells that seed the tumor bulk and may be responsible for metastases and resistance to therapy. The mechanisms regulating how lung cancer stem cells undergo self-renewal and maintain tumor growth are unknown. In recent experiments, it was determined that a small fraction of lung cancer cells asymmetrically divide their template DNA, and that the process is abrogated by Notch pathway inhibitors. This finding has broad implications in normal and tumor cell homeostasis research. The central hypothesis of this proposal is that Notch signaling regulates the lung cancer stem cell pool by maintaining a dynamic balance between asymmetric and symmetric divisions. The objectives are to determine if asymmetrically dividing lung cancer cells are restricted to the cancer stem cell fraction and if Notch signaling regulates asymmetric cell division in lung cancer. The first specific aim to achieve these objectives is to study asymmetric cell division as a fundamental mechanism restricted to the lung CSC pool. The second aim is to investigate canonical Notch signaling as a regulator of asymmetric cell division and CSC self-renewal in lung cancer. This will be achieved by expression analysis of Notch pathway genes in asymmetrically dividing human lung cancer cells, and targeting of identified proteins for disruption of asymmetric cell division and alterations in self-renewal. The rationale of the proposed research is that, once it is known that self-renewal of the CSC pool is regulated by mechanisms that control asymmetric division, those pathways can later be targeted pharmacologically by new and innovative approaches in the treatment of lung cancer.